1. Field of Invention
This invention relates generally to the fabrication of semiconductor devices and more particularly to a method of forming metallization patterns in a damaged hydrogen passivated surface layer.
2. Prior Art
Known semiconductor devices, such as photovoltaic cells, frequently comprise a plurality of conductive metallic segments attached to the front surface of a selected substrate for conducting current between various regions of the device. In the fabrication of such semiconductor devices, a plurality of relatively involved processing steps are typically required to prepare the surface of the substrate for metallization formation of the front surface conductors. These processing steps are required because the front surface of the substrate of conventional semiconductor devices is typically coated with a layer of material to which conductive materials deposited by known methods, e.g., the immersion plating process disclosed in U.S. Pat. No. 4321283, will not readily adhere. This front surface layer is typically a dielectric layer, e.g., a glass or silicon nitride layer or a damaged layer formed in the course of performing hydrogen passivation. The glass and silicon nitride layers provide hard protective coatings for the underlying silicon substrate. As discussed in U.S. Pat. No. 4691077 to Gregory et al, hydrogen passivation is used to reduce minority carrier recombination losses in polycrystalline silicon semiconductor materials.
Conventionally, metallization patterns are formed by selectively removing portions of the dielectric front surface layer using known photoresist and etching methods. Selected portions (depending on the process used) of the front surface of the substrate define the regions to which conductors will be attached. Examples of known solar cell photoresist and etching methods are taught in U.S. Pat. Nos. 4451969, 4557037, and 4612698.
Prior known methods for forming metallization patterns are not entirely satisfactory because they involve a relatively large number of processing steps. For instance, in U.S. Pat. No. 4451969, relating to the production of solar cells, the following steps are required to prepare a photovoltaic cell silicon substrate for metal plating:
(a) forming a polysilazane coating on a first surface of the substrate having a junction in the substrate adjacent the first surface; PA1 (b) covering the polysilazane coating with a photoresist material; PA1 (c) exposing the photoresist coating to radiant energy through a mask defining a two-dimensional pattern; PA1 (d) chemically developing the photoresist so that selected portions of the photoresist are removed according to the two-dimensional pattern; PA1 (e) etching away those portions of the polysilazane coating not covered by the photoresist; and PA1 (f) heating the silicon substrate at a temperature and for a time sufficient to: PA1 (a) hydrogen passivating the bulk region of a solar cell having a P/N junction formed in said cell adjacent a first surface thereof, and PA1 (b) laser annealing those portions of the damaged surface layer created by the hydrogen passivation to which conductors will subsequently be attached by immersion plating.
(1) effect removal of said photoresist material by pyrolysis, and PA2 (2) modify said polysilazane coating so that it has a relatively low etch rate.
Since the cost of semiconductor devices such as solar cells is dependent to a major extent upon the number of required fabrication steps, it is clearly desirable to be able to form metallization patterns using a process having fewer steps than the foregoing process.
Lasers have been used to remove, or alter the microstructure of various layers on substrates of semiconductor devices, in part, as a method of reducing the number of steps involved in fabricating the device. For instance, lasers have been used in conjunction with etchants to remove selected portions of semiconductor materials, as taught, for instance, in U.S. Pat. Nos. 4331504, 4401367, 4478677, 4490210, and 4490211. Lasers have also been used to alter the microstructure of portions of semiconductor materials to facilitate subsequent removal of the laser-scanned portions by specific surface etchants, as disclosed in U.S. Pat. Nos. 4335295 and 4415383. It also is known to cut holes by laser beam in thin films such as silicon nitride, copper and other conductor and semiconductor materials, as shown, for example, by U.S. Pat. Nos. Re: 27772, 4044222 and 4081653. Lasers have also been used to anneal predetermined areas of amorphous or polycrystalline semiconductor materials into single crystal areas, as disclosed by U.S. Pat. No. 4388145.
Prior known methods of altering the microstructure of electrical conductor or semiconductor materials have several disadvantages when applied to the fabrication of solar cells. First, to avoid damaging the P/N junction in photovoltaic cells, the intensity and duration of exposure to the laser beam must be carefully selected. Known laser processing methods tend not to be concerned with utilizing the laser beam in a manner preventing deterioration of the P/N junction of a solar cell. Second, in the case of U.S. Pat. No. 4388145, laser annealing tends only to be effective in those predetermined regions of the silicon substrate comprising a material substantially transparent to the laser beams. As such, the method of U.S. Pat. No. 4388145 tends to lack utility for annealing selected portions of a layer formed by hydrogen passivation of a silicon substrate.